Gangable, electric wiring boxes are widely used in domestic or industrial wiring installations. A typical gangable, electric wiring box as described for example in Canadian patents 1,071,746 and 1,083,701, and in U.S. Pat. Nos. 4,165,010 and 4,176,759, comprises a unitary U-shaped strip of sheet metal constituting the top, bottom and rear walls of the electric wiring box, and side plates removably attached to the U-shaped strip for closing its opened sides and form the side walls of the electric wiring box. The modular structure of the wiring box allows to assemble on site a wiring enclosure having the desired dimensions.
The mechanical arrangement for retaining the side plate to the U-shaped strip includes a pair of interengaging mating members forming a releasable interlock at one end of the side plate and at the opposite end, a locking screw threaded on the U-shaped strip, overlapping the side plate to achieve an interference relationship therewith, urging the side plate in intimate contact with the U-shaped strip. The locking screw engages a slanted recessed surface on the side plate, constituting an abutment for the screw and having the capability to compensate for manufacturing tolerances by providing an extended screw engaging area.
In practise, it has been found difficult to keep the dimensional relationship between the slanted surface and the length of the locking screw to the needed tolerances, resulting in a loose assembly between the side wall and the U-shaped strip when the tip of the locking screw fails to create a sufficient amount of pressure on the slanted surface. As a result, the electrical wiring box may fall apart when subjected to a strong mechanical impact, such as when it is hit with a hammer to drive fasteners for attaching the wiring box to a stud of a hollow wall structure.
An additional difficulty may develop when the installer will attempt to compensate for the insufficient screw length by driving the locking screw further until the screw head rests on the U-shaped strip. The screws which are currently used to assemble electric wiring boxes are not threaded completely up to the head. When such a screw is driven completely through the threaded hole, the screw thread in the U-shaped strip is likely to be damaged.
The problem of a loose assembly or striped threads may be solved by providing a longer screw. However, considering that the tip of the screw meets the slanted surface at an incidence angle deviating substantially from 90.degree., a camming effect is created and as a result the slanted surface cannot provide a positive and firm abutment for the locking screw. Accordingly, it is difficult to sense precisely by the amount of torque applied on the screwdriver the point at which the locking screw creates a sufficient amount of locking pressure on the slanted surface. The reduced feedback requires a considerable amount of dexterity and experience to determine the critical screw tightening point, which if exceeded, will cause a distortion of the side plate.